This invention relates to a surface emitting semiconductor laser/light emitting diode structure and, more particularly, to a high density surface emitting semiconductor structure without a substrate with independently addressable individual light emitting elements.
The ability to fabricate closely spaced, or high density, independently addressable laser and light emitting diode (LED) sources is important for many applications such as optical disk technology, laser printing and scanning, optical interconnection and fiber optic communications.
Individually, semiconductor light emitting diodes and semiconductor lasers are low power output devices. Arrays of semiconductor light emitting diodes or semiconductor lasers can be used to increase the power output and to simplify optical system design. To provide and maintain good optical alignment of the light emitting elements of the array with one another and to minimize the assembly involved, arrays have been fabricated so that the light emitting elements are in a single semiconductor substrate.
One problem with such arrays is maintaining the electrical and optical isolation between the individual light emitting elements. Another problem is increasing the density of the light emitting elements in the substrate by closely spacing the elements together while still maintaining the isolation, avoiding heat dissipation problems, and providing precise alignment of the elements.
Another problem is making each individual light emitting element in the array independently addressable. As the light emitting elements are spaced closer together in higher densities, it is progressively more difficult to separately, individually and independently cause each element to emit light.
Typical laser and LED sources have been edge emitters. The light is emitted from the edge of a monolithic structure of semiconductor layers. An alternative construction is surface emitters where the light is emitted from the surface of the monolithic structure of semiconductor layers.
Surface emitting semiconductor light sources have several advantages over edge emitters. The light emitting surface of a surface emitter is larger than that of an edge emitter. The power needed to cause emission of light is less for a surface emitter than an edge emitter. Surface emitting LEDs are more efficient than edge emitting LEDs because they have that larger emitting area. Fabrication of surface emitting lasers can be less complex than fabrication of edge emitting lasers since the cleaving and mirror passivation needed for edge emitters are eliminated.
If the individual contacts that make the surface emitters of the array independently addressable are on the same side of the semiconductor as the light emitting surface, the lateral spacing between the surface emitting light sources is restricted and consequently the density of the array is limited. The diameter of the contact will be larger than the diameter of the emitting surface. Annular contact shapes are also more difficult to contact while maintaining a clear aperture for the emitting surface. For direct wire bonds the contacts may require an auxiliary pad. Same surface contacts and emitting surfaces also may increase the amount of heat generated and may cause dissipation problems.
The individual contacts that make the surface emitters of the array independently addressable should also be directly aligned with the light emission optical cavity. Alignment is advantageous because it minimizes the electrical resistance, the current spreading to each emitter, and the size of the electrode. It also places the heatsinking as close to the emitting area as possible. Minimizing current spreading helps electrically isolate the individual elements.
Prior embodiments of the laser/light emitting diode array have all been fabricated on a substrate which remains part of the diode array structure. The substrate provides structural strength and rigidity to the complete semiconductor structure. Substrates are typically 75 to 100 microns thick which greatly increases the size of the semiconductor structure. If the laser/light emitting diode array is a surface emitter and the substrate is between the surface and the light generation layer, then the substrate must be removed by some means to expose the underlying surface emitters to permit the emission of light through the surface or the substrate must have a high bandgap and low doping level to facilitate light emission. To enable efficient heatsinking of the array, it is advantageous to mount the epitaxial layers of the diode structure in contact with the heatsink. This mounting orientation then requires light output to be obtained either through the heatsink or from the substrate side.
A method has been disclosed recently by Yablonitch et al., ("Double Heterostructure GaAs/AlGaAs Thin Film Diode Lasers on Glass Substrates", IEEE Photonics Technology Letters, Vol. 1, No. 2, pp. 41-42, February 1989) for liftoff of an epitaxially deposited semiconductor structure from a temporary substrate.
It is an object of this invention, therefore, to provide a high density array of light source surface emitters without a substrate.
It is a further object of this invention to provide a means for independent addressing of the individual laser or light emitting diode elements in this high density array.
It is yet another object of this invention to provide the independently addressable, individual contacts of the array on the opposite surface of the surface emitting semiconductor light source from the light emission surface.
It is still another object of this invention to provide that the independently addressable, individual contacts of the array are directly aligned with the light emission optical cavities and thereby maximize the density of emitters.